Countless studies identify social support as critical in protecting against the deleterious mental and physical health consequences of acute and chronic stress, yet it is unclear which types of social relationships are most beneficial or which neurobiological mechanisms are involved. We examined neural responses to separation from the natal group in six 2-year-old rhesus macaques using positron emission tomography (PET). Prior to separation, we conducted 15 ten-minute observations per subject to determine affiliative preferences (i.e. friendships). During separation, subjects were either housed alone, with grate access to a familiar (but not preferred) peer from the natal group, or with grate access to a friend. Subjects experienced each housing condition in a predetermined random order. Twenty four hours after separation, subjects were injected with 1 mCi/kg [F-18]-fluorodeoxyglucose, and after 30 minutes of conscious uptake they were anesthetized and scanned using a microPET P4 primate scanner. PET data were co-registered with structural magnetic resonance imaging. Preliminary analysis revealed greater glucose uptake in the nucleus accumbens in the “familiar peer” condition compared to the “alone” condition (GLM Repeated Measures, p = 0.02). The nucleus accumbens contains a high density of dopaminergic neurons and responds particularly to unanticipated reward. Our results suggest that the familiar peer’s presence was unexpectedly rewarding compared to that of the friend, who might have been expected to provide social support in a stressful context.